Photovoltaic panels are used for producing electrical energy from the irradiation of the sun. PV panels produce DC voltage having a variable voltage level. The DC voltage is commonly converted into DC voltage with a stabilized constant DC value or into AC voltage. The conversion to AC voltage enables feeding of the produced electrical power to an AC network such that larger scale energy production PV systems are possible.
The conversion of energy from the PV panels to either DC voltage or to AC voltage is carried out by a converter device. The converter is either a DC/DC converter or DC/AC converter which is also called an inverter. The conversion devices may include a maximum power point tracker (MPPT) which controls the internal operations of the converter to extract the maximum power available from the PV panels.
The converters of the PV system are normally in a shutdown state during the night time when the irradiation received by panels does not produce enough energy to be fed to the AC network. In large scale production of energy with PV panels, the nominal power of the converter can be in the range of hundreds of kilowatts. The losses of power converters need to be covered with the power from the panels. This means that with low output powers from the panel, it is not necessarily wise to start the converter.
One way of determining whether the operation of the converter can be started is to measure the open circuit voltage of the panel or panel system. The open circuit voltage rises as the irradiation received to the panels increases, and when the measured open circuit voltage exceeds a set limit, the operation of the converter is started.
When the temperature of the panel system rises while the irradiation stays at the same level, the open circuit voltage decreases. This means that the converter of the system is started too late. In the worst case, if the temperature rises rapidly the converter cannot be started at all. Correspondingly, when the temperature of the panel system decreases, the system can produce a higher open circuit voltage with the same irradiation intensity. This leads to starting of the converter too early, and the PV system is not able to produce electricity to the network. If the converter is started too early so that the power from the panels is not high enough, the converter can actually draw power from the network for continuing the conversion operation. Once the operation of the converter has to be stopped due to too early of a start-up, the converter is separated from the network. The repeated switching to and from the network wear down the components and can lead to premature failures.